A prior art turntable provided in universal machine tools of the kind specified above has an indexing mechanism as shown in FIG. 1, which conducts indexing of a work to be machined, not shown, (that will be simply referred to as "work" hereinbelow) mounted on the turntable "a" as follows.
Upon indexing, first of all, a piston in a clamp cylinder "b" for driving the turntable is actuated in a direction shown by arrow C so as to urge a worm "f" against a worm wheel "e" engaged with a main gear "d" adapted to rotatively drive the turntable "a" to thereby eliminate backlash between the engaged teeth, and then an indexing motor "g" is driven to rotate the main gear "d" through the above-mentioned worm "f" and worm wheel "e" so as to index the work, not shown, on the turntable "a".
After the indexing of the work, a locking cylinder "h" is actuated to drive a wedge member "i" so as to stop the axial movement of the worm "f" thereby clamping the turntable "a" at the indexing position.
The above-mentioned prior art indexing mechanism has however the following disadvantage. That is to say, because the driving system includes the worm "f" and the worm wheel "e", it is necessary to lubricate the worm "f" and the worm wheel "e" with a lubricating oil. Such lubrication needs the use of a lubricating oil having a high viscosity, which generates a considerable amount of heat during high speed running., thus causing a deformation of the turntable "a" due to the heat thus generated, thereby lowering the working accuracy.
Further, even if the indexing position of the work is set accurately, when clamping the turntable "a" by driving the wedge member "i" through the action of the locking cylinder "h" after the indexing, the wedge member "i" is urged towards the worm "f" in the axial direction thereof, thus causing such a disadvantage as changes between the indexing accuracy obtained upon positioning and that obtained upon clamping.